The present invention relates to an information device using laser light, for example, a laser printer and optical disc device.
The laser printer and optical disc device are information devices that realize highly fine picture images and high density memory utilizing the nature that laser light can be focused into a minute point. In this case, a diameter of a laser light spot is determined depending upon the wavelength of the laser light, and the shorter the wavelength, to the smaller the value of the laser light that can be focused. For this reason, shortening of the wavelength of laser light is one of the most influential means for realizing further high-resolution printing and high-density recording.
The conversions of light wavelengths by non-linear optical effects are used for shortening a wavelength of laser light. In these, a second harmonic wave generation process is well known frequency of an output light beam of a laser oscillator is doubled. In this case, since the wavelength of the second harmonic wave light becomes half of that of the output beam of the laser oscillator, the diameter of the spot of the second harmonic wave light can be readily focused to about half the size of that of an output beam of a laser oscillator. An information device in which the second harmonic wave light of an output beam of a laser oscillator is used, for example a laser printer, is shown in Japanese patent application Laid-Open No. 63-66527.
In this example, the beam output from the semi-conductor laser is irradiated onto an element of conversion of light wavelength made of a non-linear optical crystal to be converted into harmonic wave light, and the beam whose light wavelength was converted is irradiated onto a photoconductive drum as it is.
The conversion efficiency of an output beam of a laser oscillator (fundamental wave) into the second harmonic wave light is proportional to the value determined by the non-linear optical constant of the crystal for an element of conversion of light wavelength used and to the square of the energy density of the fundamental wave light. When such a crystal available at present as LiNbO.sub.3, KNbO.sub.3, KTiPO.sub.4, KH.sub.2 PO.sub.4, LiIO.sub.3, Ba.sub.2 NaNb.sub.5 O.sub.15, and so on, is used, and even if the fundamental wave light whose intensity is about several ten (10) kW/cm.sup.2 through MW/cm.sup.2 is irradiated on the crystal, the fundamental wave can not be completely converted into the second harmonic wave. Therefore, fundamental wave light remains in the second harmonic wave light after the conversion.
The application of laser light to information devices under such conditions, therefore, will not serve to realize the micronization, that is an object, of a diameter of a laser light spot, and as a result high-resolution printing and high-density recording may not be realized.
An optical disc device having an optical filter for removing a fundamental wave light component from a light beam in which the fundamental wave light mixes with the second harmonic wave light is described in Japanese patent application Laid-Open No. 61-50122. Moreover, a harmonic wave light generator comprising a semiconductor laser, a non-linear optical crystal and an optical filter for removing a fundamental wave light component from a light beam in which the fundamental wave light and the second harmonic wave light mingle together is described in Japanese patent application Laid-Open No. 63-121829. However, since the optical filters described in the above literature are made from an absorptive optical material or a dielectric multi-layer coating, and since these optical filters generally have a fundamental wave light transmission factor of 0.1% to 0.2% or more, it is difficult to sufficiently eliminate the fundamental wave light by use of these optical filters. Therefore, the use of these kinds of optical filters will not serve to realize the micronization of the spot diameter of a laser light beam, and as a result, high-resolution printing and high-density recording will not be accomplished.